1. Field of the Invention
The present invention relates to a valve timing control apparatus for a valve timing adjusting unit that adjusts timing of opening and closing an intake valve or an exhaust valve of an engine.
2. Description of Related Art
A conventional valve timing adjusting unit includes a housing (first rotor), and a vane rotor (second rotor). The housing is rotatable synchronously with one of an engine output shaft and a camshaft that opens and closes an intake valve or an exhaust valve. The vane rotor is rotatable synchronously with the other one of the output shaft and the camshaft. Also, the housing has therein advance hydraulic chambers and retard hydraulic chambers defined by vanes of the vane rotor. Then, a phase control is performed to adjust a relative rotational position (relative rotational phase) of the vane rotor relative to the housing by adjusting pressure of hydraulic oil supplied to both hydraulic chambers in order to adjust timing of opening and closing the valve.
However, in a case, where a drive source of a hydraulic pump that supplies hydraulic oil serves as the engine output shaft, the hydraulic oil may not be substantially supplied immediately after the starting of the engine. Then, the relative rotational phase may be substantially varied due to the change of the vane rotor that is subjected to variable torque (torque reversal) applied through the camshaft caused by a valve spring of the intake valve or the exhaust valve.
Thus, in the conventional apparatus described in JP-A-2002-357105 (corresponding to US20020139332), the vane rotor is provided with a lock pin, and the housing is provided with a lock hole. When a projection condition is satisfied, the lock pin is displaced from a retraction position within the vane rotor to a projection position such that the lock pin projects from the vane rotor. When the lock pin located at the projection position is fitted into or engaged with the lock hole, the relative rotational phase of the vane rotor is locked such that the vane rotor is prevented from rotating relative to the housing. As a result, if a lock control is executed, in which the relative rotational phase is controlled such that the lock pin is engaged with the lock hole, during the stopping of the engine, the relative rotational phase has bee locked accordingly at the start of the engine in the next operation. As a result, it is possible to prevent the wide change of the relative rotational phase.
Then, when it becomes possible to supply substantial amount of hydraulic oil after the engine start, the lock pin is retracted to be received in the vane rotor such that the lock of the relative rotational phase is released. Subsequently, the feed-back control is executed, in which the phase control is controlled based on a difference between the actual phase and the target phase computed in accordance with the engine operational state.
In order to detect the actual phase for the execution of the above feed-back control, it is required to obtain a reference phase (reference position). Conventionally, in the execution of the feed-back control after the release of the lock, the phase is forcibly controlled to the full retard position, and the position of the shifted phase is learned as the reference position.
In a conventional valve timing adjusting unit, it is generally designed that the phase is locked to the full retard position. However, in a recent apparatus, the phase is alternatively locked to a position between the full retard position and the full advance position, and the inventor of the present invention has found the following disadvantages in the recent apparatus.
In other words, the lock pin may fall under an immovable abnormality state, in which the lock pin becomes immovable at the projection position and thereby the lock pin is prevented from being received in the vane rotor at the retraction position. When the above learning operation is executed under the immovable abnormality state, in which the phase is locked to the full retard position, the erroneous learning will not happen. However, when the above learning operation is executed under the immovable abnormality state, in which the phase is locked to the intermediate lock position, the reference position is erroneously learned while the actual phase is erroneously at the intermediate lock position instead of the full retard position, and thereby the feed-back control may be executed based on the erroneous reference position disadvantageously.